Method of and apparatus for determining if a buried current carrying conductor is buried above predetermined minimum depth

ABSTRACT

A detector  1  for detecting a current carrying conductor has a pair of vertically spaced antennae  3, 5  to detect the magnetic radiation produced by a current carrying conductor as a result of one or both of the mains power supply or an electromagnetic signal which is induced in the conductor by a dedicated signal transmitter. The detector  1  continuously calculates the depth of a buried conductor and if the buried conductor is above a predetermined minimum depth then the detector  1  triggers an audio and/or visual alarm. By reducing the sensitivity of the antennae  3, 5 , the lateral offset range for which the alarm sounds is reduced and the location of a buried conductor can be determined.

The present invention relates to a method of and apparatus fordetermining if a buried current carrying conductor is buried above apredetermined minimum depth

Before commencing excavation or other work where electrical cables,fibre optic cables or other utilities ducts or pipes are buried, it isimportant to determine the location of such buried cables or pipes toensure that they are not damaged during the work. It is also useful tobe able to track a path of buried cables or pipes. Current carryingconductors emit electromagnetic radiation which can be detected by anelectrical antenna. If fibre optic cables or non-metallic utilitiesducts or pipes are fitted with a small electrical tracer line, analternating electrical current can be induced in the tracer line whichin turn radiates electromagnetic radiation. It is known to use detectorsto detect the electromagnetic field emitted by conductors carryingalternating current.

One type of such detector works in one of three modes. These modes areclassified as either passive or active modes, the passive modes being‘power’ mode and ‘radio’ mode. Each mode has its own frequency band ofdetection.

In power mode, the detector detects the magnetic field produced by aconductor carrying an AC mains power supply at 50/60 Hz, or the magneticfield re-radiated from a conductor as a result of a nearby cablecarrying AC power, together with higher harmonics up to about 3 KHz. Inradio mode, the detector detects very low frequency (VLF) radio energywhich is re-radiated by buried conductors. The source of the originalVLF radio signals is a plurality of VLF long wave transmitters, bothcommercial and military.

In the active mode, a signal transmitter produces an alternatingmagnetic field of known frequency and modulation, which induces acurrent in a nearby buried conductor. The signal transmitter may bedirectly connected to the conductor or, where direct connection accessis not possible, a signal transmitter may be placed near to the buriedconductor and a signal may be induced in the conductor. The buriedconductor re-radiates the signal produced by the signal transmitter.

These systems are widely available and have been marketed byRadiodetection Ltd for some time under the trade marks ‘C.A.T’ and‘Genny’.

This invention provides further advancements to existing systems,providing additional functionality and benefits to the user. Thedetector achieves good performance in terms of sensitivity, dynamicrange and selectivity. Typical parameters are 6×10⁻¹⁵ Tesla sensitivity(referred to a 1 Hz bandwidth), 141 dB rms/√Hz dynamic range, and aselectivity which allows 120 dB attenuation across a 1 Hz transitionband. The detector can be digitally programmed to receive any frequencyup to 44 kHz and processed through any defined bandwidth.

According to a first aspect of the invention there is provided a methodof determining if a buried current carrying conductor is buried above apredetermined minimum depth, the method comprising: using two magneticsensors local to the conductor and above ground to generate first andsecond respective field strength signals each proportional to thestrength of an electromagnetic field of known frequency bands producedby the conductor; sampling the first and second field strength signalsto produce first and second digitised signals respectively; processingthe first and second digitised signals to isolate respective first andsecond signals of the known frequency bands; dividing the first signalof the known frequency bands by the second signal of the known frequencybands to determine a depth signal; comparing the depth signal with apredetermined threshold value; and triggering an audio and/or visualalarm depending upon the outcome of the comparison between the depthsignal and the predetermined value.

Preferably the first and second digitised signals are simultaneouslyprocessed for two or more distinct frequency bands, the distinctfrequency bands being: (i) mains power frequency band; and (ii) apredetermined frequency band produced by a dedicated signal transmitter.

According to a second aspect of the invention there is provided adetector for detecting a buried current carrying conductor anddetermining if it is buried above a pre-determined minimum depth,comprising: two magnetic sensors for converting electromagneticradiation from the conductor into respective field strength signals,each field strength signal being proportional to the strength of anelectromagnetic field of known frequency bands produced by theconductor; electronic means arranged to sample the first and secondfield strength signals to produce first and second digitised signalsrespectively; means for processing the first and second digitisedsignals to isolate respective first and second signals of the knownfrequency bands; means for dividing the first signal of the knownfrequency bands by the second signal of the known frequency bands todetermine a depth signal; means for comparing the depth signal with apredetermined threshold value; and means for triggering an audio and/orvisual alarm depending upon the outcome of the comparison between thedepth signal and the predetermined threshold value.

Preferably the known frequency bands are selectable from the followingfrequency bands: (i) mains power frequency band; and (ii) apredetermined frequency band produced by a dedicated signal transmitterfor the detector for detecting a current carrying conductor.

Preferably the detector comprises a visual display arranged to provide avisual alarm when triggered by the triggering means.

Preferably the detector comprises an audio alarm arranged to provide anaudible alarm signal when triggered by the triggering means.

Preferably the detector comprises a manually-operable control forselecting the sensitivity of detection of the first and second fieldstrength signals so as to alter the effective horizontal offset rangefor determining whether the current carrying conductor is buried abovethe predetermined minimum depth.

According to a third aspect of the invention there is provided acomputer programme causing a computer to carry out the steps of:sampling first and second field strength signals generated by twomagnetic sensors local to a current carrying conductor and above ground,each field strength signal proportional to the strength of anelectromagnetic field of known frequency bands produced by theconductor, to produce first and second digitised signals respectively;processing the first and second digitised signals to isolate respectivefirst and second signals of the known frequency bands; dividing thefirst signal of the known frequency bands by the second signal of theknown frequency bands to determine a depth signal; comparing the depthsignal with a predetermined threshold value; and triggering an audioand/or visual alarm depending upon the outcome of the comparison betweenthe depth signal and the predetermined value.

Preferably the first and second digitised signals are simultaneouslyprocessed for two or more distinct frequency bands, the distinctfrequency bands being: (i) mains power frequency band; and (ii) apredetermined frequency band produced by a dedicated signal transmitter.

The present invention can be implemented either in hardware or insoftware in a general purpose computer. Further, the present inventioncan be implemented in a combination of hardware and software. Thepresent invention can also be implemented by a single processingapparatus or a distributed network of processing apparatuses.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of embodiments inaddition to those described and of being practiced and carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein, as well as the abstract, are for thepurpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilised as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention.

Embodiments of the invention will now be described by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is an overall block diagram of the detector for detecting aburied current carrying conductor embodying the invention;

FIG. 2 is a block diagram of the power supply unit of the detector ofFIG. 1;

FIGS. 3 a and 3 b are block diagrams of the avoidance mode system of thedetector of FIG. 1; and

FIG. 4 is a block diagram of the radio mode selectivity block of theavoidance mode system of FIG. 3 b.

Referring to FIG. 1, a detector 1 has two vertically spaced antennae,namely a bottom antenna 3 and a top antenna 5 within an elongatevertically held housing (not shown) configured to be moveable manuallyby an operator using a handle. The antennae 3, 5 are arranged with theiraxes parallel and spaced apart so that in use the bottom antenna 3 willbe directly below the top antenna 5, their axes being horizontal. Eachantenna 3, 5 produces an electrical signal which is fed into arespective one of two amplifiers 7. The amplifier outputs are fieldstrength signals 9 which are fed into a CODEC 11.

Each of the antennae 3, 5 has a noise floor. Each electrical signal fromthe antennae 3, 5 is fed to its respective amplifier 7 to lift themagnetic sensor noise floor above an intrinsic quantisation noise of theCODEC 11, this being referred to the bandwidth of one or more of thefrequency bands of detection. The output of each amplifier 7 is fed intothe CODEC 11.

The antennae 3, 5 used are high sensitivity ferrite windings. Othermagnetic sensors may be used such as Hall effect sensors or flux gatemagnetometers.

The CODEC 11 is a 24-bit stereo delta-sigma analogue to digitalconverter (ADC). This is a relatively cheap device which is commonlyused in the audio industry. In Radiodetection Limited's product marketedunder the ‘RD4000’ trade mark, pre-selective filtering, multiple switchgain stages and a phase sensitive heterodyne circuit are used betweenthe antennae and the ADC. The present invention does not usepre-selective filtering, multiple switch gain stages or a phasesensitive heterodyne circuit between the antennae and the ADC, hencereducing the number of components. In other prior art cable detectors,more sophisticated and consequently more expensive ADC are used, as theabsolute accuracy of the device measurements is important.

The CODEC 11 used in this embodiment has an absolute accuracy of ±5%,however the way that the CODEC 11 is used makes it an ideal ADC for thisapplication. High dynamic range negates the requirement for multiplegain stages. The high dynamic range is achieved by massivelyoversampling the bandwidth of detection—the noise shaping aspect of theaudio CODEC 11 being an ideal application for this principal.

Notwithstanding the poor absolute accuracy of this audio-grade stereoADC, the present embodiment benefits from the fact that the detector 1calculates the depth of a buried conductor by processing and comparingthe signals received from the two antennae 3, 5. Therefore, any absoluteinaccuracy in the sampling of the CODEC 11 is overcome by comparing thetwo processed signals. Using this CODEC 11 as a ratiometric deviceprovides a significant cost reduction, without compromising overallperformance of the detector 1.

The CODEC 11 oversamples the field strength signals 9 at up to 96 KHz.The output 13 of the CODEC 11 is fed into a digital signal processingblock 15, which is comprised of a digital signal processor 16 (DSP) anda field programmable gate array 18 (FPGA).

The DSP 16 primarily has three tasks. Firstly, it is responsible fordefining the selectivity of the detection frequency bands. Secondly, itmanages the audio and video outputs of the detector. Thirdly, itprovides general control functions to other components of the detector1.

More details of the operation of the DSP's tasks are provided inRadiodetection Limited's applications published as WO 03/071311, WO03/069598, WO 03/069769, GB 2400994 and GB 2400674, which areincorporated herein by reference.

Significant benefits are derived from ultra-narrow bandwidth processing,noise typically scaling with the square of bandwidth. The detector 1processes in several frequency bands simultaneously, allowing ballisticresponse functions, such as the general locate task, to co-exist withnarrow bandwidth functions, such as depth computation. The depthcomputation task computes in a 1 Hz bandwidth at any frequency up to 44kHz, the out-of-band rejection being around −120dB.

Phase tracking is an essential feature to allow the narrow bandwidthtasks to lock-on to the carrier frequency, the potential error betweentransmitter and receiver clocks being well in excess of the signalbandwidth. In the case of the active mode, the transmitted signal may be100% amplitude modulated and the depth calculation task has to positionitself exactly on the carrier without cross-talk from the side-bands(located at ±6 Hz around the 32,768 Hz carrier).

The phase tracking algorithm is a natural development of processesdescribed in Radiodetection Limited's UK application no. 0407372.2.Essential signal to noise ratio (SNR) measurements are made on thecarrier and side-bands and checks performed to ensure the trackingalgorithm does not wander off on any high order harmonics due topower-line transmissions. SNR is quantified from both magnitude andsecond derivative phase information; all results are correlated fromboth antennae 3, 5. In the case of an SNR less than 10 dB, the depthcalculation task is disabled, thus ensuring only accurate information ispresented to the user.

The concept of spectral recognition is applied to the active signal whenit is in pulsed mode operation. This idea is a simple application of thealgorithms described in Radiodetection Limited's UK application no.0407372.2 and involves a spectral assessment of the carrier and AMside-bands. The assessment is a Discrete Fourier Transform (DFT)convolution and measurement of the SNR. The DFT itself moves with thetracking algorithm and locks on to the carrier frequency.

The combination of these methods ensures that the detector 1 achievesthe best possible signal integrity and depth accuracy.

User control of the detector 1 is provided by means of a sensitivitycontrol 17 and a switch 19. The switch 19 is used to set the mode ofoperation of the detector 1. For example, the detector 1 can be set tooperate in radio, power or active mode. The active mode is chosen when adedicated signal generator is used in proximity to the cable which is tobe detected, the signal generator inducing an alternating current in theconductor which re-radiates a magnetic signal. The signal generatoroperates at a preset frequency and with a preset modulation which isidentified by the detector 1. A further position of the switch 19 is‘avoidance’ mode, the operation of which is explained below.

The sensitivity control 17 is used to vary the gradient sensitivity ofthe antennae 3, 5. High sensitivity is initially used to detect thepresence of a weak signal produced by a current carrying conductor. Oncethe presence of a conductor has been established, the sensitivitycontrol 17 is varied to decrease the sensitivity of the detector 1 andthe detector 1 is used to more precisely determine the location of theconcealed current carrying conductor. This method of profiling thelocate window as a function of sensitivity is described inRadiodetection Limited's application published as U.S. Pat. No.6,777,923, which is incorporated herein by reference.

A liquid crystal display (LCD) 21 is provided in the housing surface todisplay such information as the mode of operation of the detector, thebattery status, the depth of a conductor and/or the strength of thedetected signal. Other user display devices can be used, as will beapparent to the skilled person.

The detector 1 also comprises a flash ROM 23, in which software isstored, and a power supply unit (PSU) 25. A key requirement of thedetector 1 is that it must be portable.

Therefore, batteries 26 are used to power the detector 1, in this casetwo ‘D’-type batteries, each providing a nominal 1.5V.

In use, the detector 1 is powered up and software is loaded from theflash ROM 23 into the digital signal processing block 15. A user adjuststhe switch 19 to select the mode of operation. The selection will beeither radio mode, power mode, active mode or avoidance mode. A depththreshold alarm function is active in power mode, active mode andavoidance mode. In avoidance mode the depth threshold alarm functiononly operates on frequencies in the frequency bands of power mode andactive mode. The depth threshold alarm function is detailed below.

When the detector 1 is in proximity to a current carrying conductor, acurrent is induced in the bottom and top antennae 3, 5. The currentinduced in each of the antennae 3, 5 is amplified by a respectiveamplifier 7. The outputs 9 from the amplifiers 7 are field strengthsignals of the two antennae 3, 5. These signals are input to the CODEC11 which samples these signals at up to 96 kilo samples per second. Thedigitised signals 13 are fed to the digital signal processing block 15.The DSP 16 of the digital signal processing block 15 isolates signals oftarget frequency bands, depending on the mode of operation. If the DSPdetects the presence of a current carrying conductor an audio and/orvisual alarm is triggered on the speaker 22 and/or indicator 21.

Referring to FIG. 2, the PSU 25 has been designed to reduceself-generated noise which would otherwise interfere with the locatesensitivity and selectivity of the detector 1. The interferencemechanism is either conducted on the power rails or radiated as amagnetic field. The space constraints of the detector 1 mean that thereis an inevitable coupling of unwanted signals from the auxiliaryelectronics into the antennae. By carefully managing the electromagneticemissions of the PSU 25, various benefits are achieved. For example,prior art digital detectors have operated as heterodyne orsuper-heterodyne receivers, wherein the bandwidth of operation of theDSP is shifted away from the baseband signal. This shifting is requiredin order to avoid electromagnetic interference between the auxiliaryelectronics and the signal detectors and employs a significantproportion of the processing capacity of the DSP. In the presentinvention, as this capacity has been freed up by operating the detector1 as a homodyne receiver, the surplus capacity is used for otherfunctions, as is explained below.

The PSU 25 is a switched mode power supply which is managed by amicroprocessor, in this case the DSP 16. The PSU 25 provides regulatedrails at 12 V, +3.3 V and −3.3 V. The input voltage of the PSU 25decreases as the batteries 26 deteriorate. The load 31 is dynamic,primarily due to the varying current drawn by the speaker 22. Thespeaker output varies greatly as a current carrying conductor isdetected. The current drawn by the other components also fluctuates.

The PSU 25 is comprised of a pulse width modulator (PWM) 27 which iscontrolled by the DSP 16. The output of the PWM 27 is fed into a singleended primary induction controller (SEPIC) 29 which is driven at exactlyfour times the CODEC sampling frequency, i.e., at up to 384 KHz. Thisensures that the primary harmonic falls on a natural zero of the CODEC11 and DSP 16, as is known in the art. The three regulated railsproduced by the SEPIC 29 feed the dynamic load 31 of the detector 1,i.e., the components of the detector 1.

The SEPIC 29 is controlled on both edges so that the DSP 16 has fullauthority control on all of the electromagnetic emissions produced bythe PSU 25. In this way, the DSP 16 is able to eliminate any unwantedpower harmonics which would overlap the frequency bands of detection.

A proportional integral differential (PID) controller 33 feedbackalgorithm is used to control the PSU 25. The feedback bandwidth isconstrained by the requirements of noise avoidance so that none of thehigher order power switching harmonics interfere with the locatefrequency bands. The regulated voltages are filtered by a filter 39 andare fed into the PID controller 33. The outputs of the PID controller 33are combined and are provided as an input to a lookup table 35. Thevoltage of the batteries 26 is also provided as an input to the lookuptable 35. A further contribution to the control function can be a loadpredictor which offsets the duty cycle of the SEPIC 29 in response to anabrupt change in load, typically an increased audio demand. This loadpredictor function is provided by the DSP 16 which has knowledge of theload which will be required by some of the components, in particular thespeaker 22. The load drawn from the PSU 25 typically varies between 600and 1500 mW.

The inputs to the lookup table 35 are the battery supply voltage, theload predictor and up to three feedback contributions. The output is theduty cycle of the primary switch. The purpose of the lookup table 35 isto ensure that the spectral components of the PSU 25 that result fromthe regulation process cannot overlap the frequency bands of detection.This results in a discontinuous function which may hop from one dutycycle to another as an abrupt change rather than a continuous greyscaleof regulation. The specific nature of the lookup table 35 is tailored tothe mode of operation.

By using a PID controller 33 and a lookup table 35 the need for aproprietary switching regulator is eliminated, thereby reducing the costof the detector 1. The SEPIC 29 is a switching converter the output ofwhich is almost a linear function of the duty cycle. The feedbackcontrol law 33 is constrained to a bandwidth of roughly 1.5 KHz. Aboot-strap oscillator is needed to start the SEPIC 29 before the DSP 16can take control.

In alternative embodiments other combinations of PID feedback mixing maybe used. For example, the feedback law 33 may be a standard proportionalcontrol (with hysteresis), i.e., with zero integral and derivativefeedback gain. The control algorithm used is dependent on the mode ofoperation.

By controlling the self-generated noise of the PSU 25, the detectorbenefits from an improved SNR, thereby improving the sensitivity andselectivity of the detector 1.

FIGS. 3 a and 3 b shows a more detailed block diagram of the detector 1showing the ‘avoidance’ mode system, which is implemented in thedetector 1. As mentioned above, three dedicated operating modes areavailable, namely one active and two passive modes. When existingdetectors are used to check an area for a buried conductor, it isnecessary to sweep the area three times, each time with the detector ina different mode.

The detector 1 of this invention combines the dedicated active andpassive operating modes into a single mode, known as avoidance mode, tolocate buried conductors in a single sweep, thereby saving time. Ifnecessary, one or more of the dedicated modes can be used at a laterstage to identify the exact location of a buried conductor. The threemodes operate simultaneously, sharing the pair of antennae 3, 5 and acommon detection indicator 21 and speaker 22. The detection sensitivity30 is normally set to maximum, but can be set at a lower level.

The CODEC 11 is clocked at 73.242 KHZ. The DSP 16 processes the fieldstrength signals produced by the antennae 3, 5 and simultaneouslyisolates signals of each of the three frequency bands corresponding tothe three modes of operation in three mode selectivity blocks 41, 43,45.

In existing detectors, the DSP is only capable of processing one mode ata time, primarily due to processing and power constraints, and theirheterodyne architecture. However, according to this embodiment the DSP16 is able to process each of the signals simultaneously due to thecapacity which has been freed up as a result of efficiencies elsewherein the system, for example, the ability of the detector 1 to operate asa homodyne receiver.

Signal outputs from the DSP 16 corresponding to the different modes ofoperation are fed into automatic gain controllers 47 (AGCs), such as theAGC described in Radiodetection Limited's application published as U.S.Pat. No. 6,777,923, which is incorporated herein by reference. Theoutput of each of the AGCs 47 is converted to a detection signal incomparators 49. The detection signals are combined and used to providean audio output from a speaker 22 and/or a visual signal on an indicator21, for example on the LCD.

The detector 1 continually calculates the estimated depth of a buriedconductor. If the depth of a buried conductor is calculated as less thana preset threshold, e.g. 30 cm, an audio and/or visual alarm istriggered to alert the operator of a shallow conductor. Such shallowconductors are of particular interest as there is an increased risk ofhitting a shallow conductor when excavating an area.

In order to optimise the user interface of the detector, whencalculating the depth of a conductor, the DSP 16 processes signals inthree frequency bands simultaneously to tailor the manner in whichinformation is presented to the user. The depth of the conductor iscalculated in a 1 Hz bandwidth; the visual display is processed in a 10Hz bandwidth so that the flicker of the display is at an acceptablelevel; and the processing of the audio alert is performed at 35 Hz, toensure that the pulsing tone is clearly audible.

This depth threshold alarm function is active in the power mode andactive mode of operation. It is also active in the avoidance mode butonly operates for the frequency bands used in the power and activemodes. The depth of a buried cable is calculated by comparing thestrength of the signals received at the two antennae 3, 5 as shownbelow.

The bottom antenna signal E_(b)(w) and top antenna signal E_(t)(w) as afunction of horizontal offset w of the detector from the conductor aregiven by:

${E_{b}(w)}\text{:}{= {\frac{k \cdot a}{a^{2} + w^{2}}\mspace{14mu}{and}\mspace{14mu}{E_{t}(w)}\text{:} = \frac{k( {x + a} )}{W^{2} + ( {x + a} )^{2}}}}$where k is a magnetic constant; x is the distance between the antennae;and a is the vertical distance above the current carrying conductor. Thedepth of the current carrying conductor is given by:

${{d(w)}\text{:}} = {\frac{x}{\frac{E_{b}(w)}{E_{t}(w)} - 1}}$The depth threshold alarm function is given by:

${{{SA}(w)}\text{:}} = | \begin{matrix}1 & {{{if}\mspace{14mu}{\frac{E_{b}(w)}{E_{t}(w)}}} \geq T_{d}} \\0 & {otherwise}\end{matrix} $where T_(d) is the depth threshold constant which is dependent on thedesired depth above which the alarm is triggered.

If SA(w)=1, the audio and/or visual alarm is triggered. A more accuratedepth measurement can then be achieved by accurately pinpointing thelocation of the buried conductor by altering the sensitivity of thedetector 1, as described above.

This method involves a careful interleaving of the depth threshold alarmfunction into the locate profile as governed by the depth a andhorizontal offset w of the conductor and the sensitivity of the detector1.

When the detector 1 is located directly above a current carryingconductor, the difference between the signals induced in the antennae 3,5 is a maximum. As the detector 1 is moved away from the conductor themagnitude of the difference initially falls off and then rises again toa second peak. This is the case as the detector is moved in eitherdirection perpendicular to the conductor. Hence, there is a main peak inthe difference between the signals induced in the antennae 3, 5 when thedetector 1 is directly above a current carrying conductor and there aretwo smaller peaks when the detector 1 is horizontally displaced from theconductor.

It is possible that when a detector 1 evaluates the depth thresholdalarm function, the depth alarm may be triggered when the detector 1 isdirectly above the conductor and when the detector is moved either sideof the detector 1, coinciding with the smaller peaks which arehorizontally displaced from the conductor. An experienced user candistinguish between the main central peak and the two smaller sidepeaks, by moving the detector 1 through each of the three locations inwhich the depth threshold alarm is triggered, as the central locationcorresponds to the position above the conductor.

Conventionally, radio mode uses a beat frequency oscillator (BFO) tocentre the bandwidth of detection on the target VLF spectrum. Thespecific frequencies at which VLF transmissions are transmitted varyfrom one country to another. The conventional approach requires the BFOto be tuned to a specific frequency dependent on the geographicallocation.

This embodiment achieves a ‘universal’ radio mode by combining theoutputs of signals processed through a plurality of BFOs. The advantageof this approach is that the detector 1 works in a large number ofcountries and can be provided in these countries without the need forlocal configuration, thereby saving on cost and time of deployment. Thecombined BFO approach has been achieved without a loss of performance.

The source of the inputs of the radio mode selectivity block is energyfrom VLF transmission stations in the frequency band 16 KHz to 39 KHz.Referring to FIG. 4, the block 41 superimposes signals 55 from aplurality of BFOs 53 into a common algorithm, thus encompassing theentire spectrum without loss of performance in the referred signal tonoise ratio. This algorithm is very similar to all previous algorithmsother than the concept of having a plurality of BFOs. The BFOs 53 sum ona common mode junction 57, the output of which is multiplied with thesignal 59 that is output from the CODEC 11. A low pass filter 61determines the overall bandwidth detection, which is typically 10 KHz,and also rejects the trigonometric sum term which is intrinsic to themodulation.

If two BFOs 53 are used, their frequencies are chosen within the rangesof 17536 Hz to 20992 Hz and 20992 Hz to 24448 Hz. Preferably theirfrequencies are chosen within the ranges of 18400 Hz to 20128 Hz and21856 Hz to 23584 Hz. In this embodiment the frequencies of the two BFOs53 are chosen in the middle of these ranges, i.e. at 19264 Hz and 22720Hz. A further preferred frequency range is around 24700 Hz and furtherBFO frequencies may be selected to provide improved local coverage. Byoperating the plurality of BFOs 53 at these carefully selectedfrequencies, the detector can detect in radio mode in a large number ofcountries.

The remainder of the signal processing of the radio mode selectivityblock 41 is unchanged from previous architectures for processing radiomode signals. This comprises a rectifier 63, subsequently a low-passfilter 65, a down-sampling stage 67 and a further low pass filter 69.This cascaded down sampling and low pass filtering exists to decimatethe bandwidth down from the sampling rate of roughly 73 kilo samples persecond to about 610 samples per second, with an overall responsebandwidth close to 10 Hz, that being the ballistic response bandwidthfor radio mode.

Various modifications will be apparent to those in the art and it isdesired to include all such modifications as fall within the scope ofthe accompanying claims.

1. A method of determining if a buried current carrying conductor isburied above a predetermined minimum depth, the method comprising: usingtwo magnetic sensors local to the conductor and above ground to generatefirst and second respective field strength signals each proportional tothe strength of an electromagnetic field of known frequency bandsproduced by the conductor; sampling the first and second field strengthsignals to produce first and second digitised signals respectively;processing the first and second digitised signals to isolate respectivefirst and second signals of the known frequency bands; dividing thefirst signal of the known frequency bands by the second signal of theknown frequency bands to determine a depth signal; comparing the depthsignal with a predetermined threshold value to determine if the buriedcurrent carrying conductor is buried above the predetermined minimumdepth; and triggering an audio and/or visual alarm depending upon theoutcome of the comparison between the depth signal and the predeterminedvalue.
 2. A method as claimed in claim 1, wherein the first and seconddigitised signals are simultaneously processed for two or more distinctfrequency bands, the distinct frequency bands being: (i) mains powerfrequency band; and (ii) a predetermined frequency band produced by adedicated signal transmitter.
 3. A detector for detecting a buriedcurrent carrying conductor and determining if it is buried above apre-determined minimum depth, comprising: two magnetic sensors forconverting electromagnetic radiation from the conductor into respectivefield strength signals, each field strength signal being proportional tothe strength of an electromagnetic field of known frequency bandsproduced by the conductor; electronic means arranged to sample the firstand second field strength signals to produce first and second digitisedsignals respectively; means for processing the first and seconddigitised signals to isolate respective first and second signals of theknown frequency bands; means for dividing the first signal of the knownfrequency bands by the second signal of the known frequency bands todetermine a depth signal; means for comparing the depth signal with apredetermined threshold value to determined if the buried currentcarrying conductor is buried above the predetermined minimum depth; andmeans for triggering an audio and/or visual alarm depending upon theoutcome of the comparison between the depth signal and the predeterminedthreshold value.
 4. A detector according to claim 3, in which the knownfrequency bands are selectable from the following frequency bands: (i)mains power frequency band; and (ii) a predetermined frequency bandproduced by a dedicated signal transmitter for the detector fordetecting a current carrying conductor.
 5. A detector according to claim3, comprising a visual display arranged to provide a visual alarm whentriggered by the triggering means.
 6. A detector according to claim 3,comprising an audio alarm arranged to provide an audible alarm signalwhen triggered by the triggering means.
 7. A detector according to claim3, comprising a manually-operable control for selecting the sensitivityof detection of the first and second field strength signals so as toalter the effective horizontal offset range for determining whether thecurrent carrying conductor is buried above the predetermined minimumdepth.
 8. A computer-readable medium tangibly embodying a program ofmachine-readable steps executable by a processor to determine if aburied current carrying conductor is buried above a predeterminedminimum depth; the steps comprising; sampling first and second fieldstrength signals generated by two magnetic sensors local to a currentcarrying conductor and above ground, each field strength signalproportional to the strength of an electromagnetic field of knownfrequency bands produced by the conductor, to produce first and seconddigitised signals respectively; processing the first and seconddigitised signals to isolate respective first and second signals of theknown frequency bands; dividing the first signal of the known frequencybands by the second signal of the known frequency bands to determine adepth signal; comparing the depth signal with a predetermined thresholdvalue to determined if the buried current carrying conductor is buriedabove the predetermined minimum depth; and triggering an audio and/orvisual alarm depending upon the outcome of the comparison between thedepth signal and the predetermined value.
 9. The computer-readablemedium tangibly embodying program steps as claimed in claim 8, whereinthe first and second digitised signals are simultaneously processed fortwo or more distinct frequency bands, the distinct frequency bandsbeing: (i) mains power frequency band; and (ii) a predeterminedfrequency band produced by a dedicated signal transmitter.